The present invention is directed generally to mobile antennas, and more specifically to a window-mounted mobile antenna which provides improved performance and economy of manufacture in cellular telephone systems or the like.
With the advent of cellular mobile telephones the need has arisen for antennas which mount on the window of a vehicle, thereby avoiding the need to drill holes in or otherwise modify the vehicle body. Preferably, such window-mounted antennas should offer a degree of performance comparable with body mounted antennas and should be economical to manufacture and easy to install.
Typically, window-mounted antennas include an outside module on the outside of the window glass on which a generally vertical radiating element is mounted, and an inside module on the inside of the glass in registration with the outside module which contains an impedance matching circuit and in some instances a ground plane necessary for operation of the antenna.
For optimum range in cellular mobile telephone applications, it is desirable that the radiation angle of the antenna be low. To this end, it is preferable that the radiating element of the antenna have an electrical length greater than 1/2 wavelength at the antenna operating frequency In practice, a length of 5/8 wavelength has been found to be optimum in that this length provides a high degree of radiation efficiency and a very low angle of radiation. Furthermore, this length provides a reduced relatively low capacitive base reactance, which can be readily matched to a low impedance coaxial feed line with a single inductor, unlike the inductive reactance of higher base impedance half wave radiating elements.
The use of a 5/8 wavelength radiating element requires that the antenna incorporate in its inside module a ground plane to provide an electrical reference plane and impedance matching means for matching the base impedance of the radiating element to the coaxial feedline. Preferably, this inside module should be as compact and economical to manufacture, and should not require any adjustments on the part of the user. Furthermore, compensation for the capacitive reactance of the capacitive coupling through the glass window should also be accomplished without user-adjustment.
The present invention provides a new and improved window-mounted mobile antenna utilizing a 5/8 wavelength radiating element which meets the above requirement.